U.S. Pat. No. 3,868,933 discloses controlling an idle air actuator mounted in a bypass for only as long as idle is present. However, as soon as the throttle flap in the main intake channel opens, the idle air actuator is switched so that it is without current whereupon it assumes a base position, preferably a center position. The idle air actuator is again driven when the idle is reached or when a resume engine speed is reached for overrun cutoff.
This method has the disadvantage that unevenness in the air flow occurs during current cutoff as well as when the idle air actuator is again switched on. This unevenness in the air flow negatively influences the driving performance of an internal combustion engine driven pursuant to this method.
Accordingly, it has been the basic practice to continuously drive the idle air actuator whereby the condition has been fully precluded that switch-off and switch-on disturbances occur. In refined methods, the control which is present when leaving the last idle condition is not continuously maintained; instead, the drive value is modified in dependence upon changes in the operating conditions. If the idle condition were left for the last time, for example, when the engine was relatively cold, and then the engine temperature increases because of the continued operation thereof, the control corresponding to the changed engine temperature would change in the direction of reduced air throughput through the bypass such that, when idle is reached, the desired idle engine speed is again adjusted. If deviations from the desired engine speed occur, then these are compensated for by an idle control. It should now be noted that refined systems consider not only the idle case but also the resume case during overrun cutoff. In an operation of this kind, a somewhat larger air cross section in the bypass is provided than in the idle case. The basis for the corresponding drive of the idle air actuator is however the drive value present when the idle condition was last left as this value was modified because of changes in the operating conditions.
The continuous drive of the idle air actuator leads to power losses which are considerably higher than, for example, the driving of the injection valves. The power loss, which is caused by the driving of the idle air actuator, amounts to up to 25% of the total power loss of a control apparatus. The relatively high power loss must be considered with the construction of the control apparatus pursuant to spatial distribution and usable components. All these disadvantages have however been accepted for years in order to avoid the above-mentioned other disadvantages which occur when the idle air actuator is switched to zero current when leaving idle and driven only when reaching idle or the resume engine speed with overrun cutoff.